The continuously increasing demand for higher bandwidth in the actual non-licensed wireless communication systems, aiming to transmit high resolution video streams which need data rates in the multi-GBbps ranges forced a search for frequency ranges where such a bandwidth is available like the unlicensed 60 GHz ISM band also known as millimetre wave band (mm-wave) or other unlicensed frequency bands.
The several GHz of bandwidth available in these bands, however, is accompanied by a severe free space path loss which limits the operation range. Furthermore, mm-waves do not pass through objects or people. Hence, mm-waves will not penetrate walls and a person passing through a 60 GHz link will also block the transmission completely. A wireless network based on this technology, e.g. to distribute high definition video content into a household, needs to hop over one or more intermediate nodes to reach a destination placed in another room.
To compensate the path losses, the radios in the wireless terminals running at these high frequencies employ one or multiple beam directional antennas. These type of antennas are capable to orientate one or more antenna beams in a specific direction (within the physical limits of the specific antenna) by steering the main beam in that direction (beam steering antennas) or by switching on the beam with the closest orientation to the desired one (beam switching antennas). The benefits are threefold:    1. The directional antenna gain compensates for the propagation losses    2. Thanks to a beam selection algorithm the radio link control (RLC) procedure has detailed information about the channel conditions which can analyse to select the best antenna beams combination between transmitter and receiver.    3. Additionally, a beam tracking algorithm keeps the RLC continuously informed about the channel conditions of the active antenna beam combination and the other possible ones.
Some beam selection and tracking algorithms proposed in the literature work under the assumption that the channel for every antenna beam combination can be sounded/measured at any time using a specific PHY frame sent by a transmitter. These frames are used in the receiver to estimate several characteristics of the wireless channel like the power delay profile, SNR, etc. Such characteristics are then employed by the beam selection or tracking algorithm to calculate one or more parameters which directly relate to the channel quality for the specific beam combination (link).
However, the main problem with above approach consists in how to reliably sending back to the transmitter the measurements done at the receiver for every antenna beam combination in order to run the beam selection and tracking algorithms effectively. The timely provision of this information on the transmitting side is critical for seamlessly switching to another antenna beam combination in case of a sudden heavy degradation (e.g. temporal obstructions) of the channel quality.
On the other hand, even if no obstruction or any other channel degradation occurs in the directional data channel, sending additional control information over the same mm-wave channel can cause excessive delays if a retransmission of data is necessary, eventually blocking the content source transmission. This is particularly an issue in high definition content streaming applications which are very sensitive to delays, since some control information has to be exchanged between the data source and the data consumer like HDCP (High bandwidth Digital Content Protection).
Moreover, using the same mm-wave radio to send the measurements information back to the transmitter also forces the wireless terminals to implement a complete mm-wave radio transceiver (transmitter and receiver), although most of the devices this multi-Gbps technology is intended to are usually either data producers (Blue-Ray player, HD Camcorders, Game consoles with HD output, still cameras and so forth), or data consumers (TV, Beamer and so forth). Therefore, implementing an out-of-band back channel would save resources in the mm-wave radios: instead of complete radio transceivers, the data producers would implement only a mm-wave transmitter and the data consumers only a receiver.
Therefore, for at least the above reasons, it is not recommendable to use the same antenna link for sending back control and measurement data, because this channel may not be very robust due to sudden obstructions, the sent back control data may create unacceptable delays in streaming the user data (e.g. movie) and lastly, subjects consumer electronic devices to the heavy burden of implementing costly two way radios.